The present invention relates to load-lifting masts for load lifting and transporting vehicles. Although not limited to use with automatically-guided vehicles, it is especially adapted to compensate for the absence of a driver in such vehicles.
Load-lifting masts of both the screw-driven type and the hydraulically-driven type have long been used on driver-type industrial trucks and, more recently, on automatically-guided vehicles. Some of these masts have been equipped with automatic sensors of various types.
For example, mast slack chain sensors have been used for safety reasons to interrupt further lowering of a mast to prevent sudden load drop, as shown in Gandolfo U.S. Pat. Nos. 3,224,529 and 3,416,109, Branham U.S. Pat. No. 3,612,221 and Luebrecht et al. U.S. Pat. No. 4,499,971. However, such sensors have not been used to solve the unrelated problem of proper load depositing by automatically-guided vehicles. Normally such vehicles control load depositing solely by comparing sensed carriage height to a target height, operating on the assumption that there is a supporting surface at the target height which can accept the deposited load. However, if the load is not yet fully supported when lowered to the target height, or becomes fully supported before being lowered to such height, serious load depositing malfunctions can result from such reliance on height sensing. For example, when stacking loads on top of other loads, the proper height for load depositing can vary dramatically with time due to load compression, temperature and humidity conditions, such that a system for depositing loads which is referenced solely to predetermined heights would be unworkable.
Carriage height sensors have long been used on load-lifting masts for automatically and nonautomatically-guided vehicles alike, as shown for example in Rutledge U.S. Pat. No. 3,818,302, Tjoernemark U.S. Pat. No. 4,130,183, Melocik U.S. Pat. No. 4,206,829, Dammeyer U.S. Pat. Nos. 4,265,337 and 4,280,205, Nakada U.S. Pat. No. 4,411,582 and Schultz U.S. Pat. No. 4,598,797. The problem with all such height sensors, however, is in maintaining their accuracy. Inaccuracies develop rapidly in such sensors because of the lifting mechanisms themselves, which are susceptible to wear, chain stretch and maladjustment due to the heavy usage which they experience. Moreover, the relationship of the mechanism with respect to the ground or other vehicle-supporting surface also varies due to such factors as tire wear. All of these factors result in the frequent, recurring introduction of error into carriage height sensor readings. Such errors can be temporarily corrected by manual recalibration of the sensors, but this is far too time-consuming to be done while the mast is in use. Where the mast is mounted on a driver-type vehicle, such errors may not be particularly critical since the driver can compensate for them. However, where the mast is mounted on an automatically-guided vehicle, the continuous accuracy of height sensor readings is critical, and any frequently recurring errors are therefore unacceptable.
Likewise, because the continued ability of the mast to lift and hold a load on command are vital to an automatically-guided vehicle having no driver to notice and correct malfunctions, testing of such functions should be carried out on a relatively continuous basis during use of the mast, rather than on an intermittent service basis as is normal. Although Melocik et al. U.S. Pat. No. 4,567,757 recognizes the importance of such testing with respect to the operability of automatically-guided vehicle brakes, neither the need nor the means for automatic testing of load-lifting mast functions while in use has been previously suggested.
The preferable powered lifting mechanism for an automatically-guided vehicle mast is a vertically-oriented screw member rotatably driven so as to reciprocate a drive nut vertically. However, interfacing such a screw member with a load-lifting mast presents problems caused by the unsymmetrical loading of the mast. The mast will virtually always be subjected to a forward and downward load moment due to the forward protrusion of the load relative to the mast and, if the load is not centered with respect to the mast, will experience side moments as well. Moreover, horizontal forces in both fore-and-aft and transverse directions are to be expected in the handling of loads. Such moments and forces, if transmitted to the nut and screw member, can cause damaging warping and wear, detracting from the needed accuracy and reliability of the mast. Although some trunnion-type interfaces, such as that shown in Olsen U.S. Pat. No. 3,568,804, have been developed for isolating vertical screw members from the moments and side forces imposed by their loads, such interfaces depend largely on tensile forces rather than compressive forces to lift the load, and their structures are therefore generally not strong enough to accept the degree of loading normally imposed upon a load-lifting mast.
An optimum mast for an electrically-powered vehicle, such as a battery-powered driver-type lift truck or automatically-guided vehicle, should employ the most efficient of power controllers, preferably field-effect transistors (FETs), for regulating its electric lift motor. However, the large variations in voltage which characterize battery power sources, due to variations in loading and charging state, present difficulties in the utilization of FETs because of the need for predetermined differences between source voltage and gate voltage to enable an FET to be turned on. Prior FET control circuits, such as that shown in Damiano U.S. Pat. No. 4,599,555, recognize this problem but solve it by means of relatively complicated gate voltage control circuitry. A much simpler gate control system is needed to facilitate the economical utilization of FETs for power control where source voltage is expected to vary substantially, not only for load-lifting masts but for all applications.